Abstract: Based on ionic theory, a constitutional model is established for calculation of activity of CaO in basic slag. The concept of gross interaction effect and self-action effect of CaO in melten slag is also presented, and the gross interaction coefficient and self-action coefficient of CaO are determined from the experimental data after Talor and Chipman. Finally, the model is checked by the activities of CaO measured by experiments in different slag systems. The results show that the activity of CaO calculated from the model well agrees with the experimental data.